Method and device for ambient light measurement

ABSTRACT

An embodiment method of measuring ambient light comprises generating, by an ambient light sensor associated with a screen which alternates between first phases in which light is emitted and second phases in which no light is emitted by the screen, a first signal representative of an intensity of light received by the ambient light sensor during the first and second phases; comparing the first signal with a threshold intensity value; and controlling a timing of an ambient light measurement by the light sensor based on the comparison.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of European Application No.20153603.4, filed on Jan. 24, 2020, which application is herebyincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to electronics systems andmethods, and more specifically to electronic systems that comprise anambient light sensor and to methods of measuring ambient light with suchsystems.

BACKGROUND

Electronic systems such as mobile telephones or tablets comprisingscreens displaying information and/or images destined for a user of thesystem are known.

In such systems, the light power emitted by the screen can be at leastpartly adapted as a function of the level of ambient light, this levelof ambient light being measured by means of an ambient light sensor(ALS). For example, this ambient light measurement is used to adjust thelight power emitted by the screen as a function of the level of ambientlight for a better perception of image displayed on the screen by thehuman eye, as well as to save energy, and thus extend the life of abattery supplying the screen.

In known electronic systems comprising a screen and an ambient lightsensor for measuring the intensity of the surrounding or ambient light,the sensor is disposed under a protective glass covering the screen,under a dedicated opening in the screen. It would be desirable toposition the sensor under the screen, without a dedicated opening in thescreen, the sensor capturing the weak transmission of the light throughthe screen. However, it is then difficult for the sensor to distinguishwith precision the light emitted by the screen in the direction of thesensor from the ambient light passing through the screen from theexterior to the sensor.

SUMMARY

There is a need to address all or some of the drawbacks of the knownelectronic systems comprising a screen and a light sensor for measuringthe level of ambient light surrounding the system.

Thus, one embodiment addresses all or some of the drawbacks of the knownelectronic systems comprising a screen and a light sensor for measuringthe level of ambient light surrounding the system.

In particular, one embodiment makes it possible to avoid that the levelof ambient light measured by the light sensor is distorted by the lightemitted by the screen.

One embodiment provides a method of measuring ambient light comprising:generating, by an ambient light sensor associated with a screen whichalternates between first phases in which light is emitted by the screen,a part of which is received by the ambient light sensor, and secondphases in which no light is emitted by the screen, a first signalrepresentative of an intensity of light received by the ambient lightsensor during the first and second phases; comparing the first signalwith a threshold intensity value; and controlling a timing of an ambientlight measurement by the light sensor based on the comparison.

According to an embodiment, comparing the first signal with thethreshold intensity value comprises generating a second signal having afirst state when the intensity of the light received by the ambientlight sensor is below the threshold intensity value, and a second statewhen the intensity of light received by the ambient light sensor isabove the threshold intensity value, and wherein a start of themeasurement is triggered by at least one transition of the second signalto the first state such that the measurement starts when the secondsignal is in the first state, and a duration of the measurement iscontrolled based on at least one duration of the first state of thesecond signal.

According to an embodiment, the measurement starts following a firsttransition of the second signal to the first state.

According to an embodiment, the measurement starts a time delay between0.1 μs and 100 μs after the first transition.

According to an embodiment, the first transition triggers the start ofthe measurement, and a next transition of the second signal to thesecond state following the first transition ends the measurement.

According to an embodiment, the duration of the measurement iscontrolled based on at least one previous duration of the first state ofthe second signal.

According to an embodiment, the duration of the measurement iscontrolled to be shorter than the at least one previous duration of thefirst state of the second signal.

According to an embodiment, the duration of the measurement iscontrolled based on an average value of at least two successive previousdurations of the first state of the second signal.

According to an embodiment, the at least one previous duration of thefirst state of the second signal comprises the previous duration of thefirst state of the second signal immediately preceding the first stateof the second signal during which the measurement starts.

According to an embodiment, the method comprises counting a number ofclock cycles with a counter during the at least one previous duration ofthe first state of the second signal, the duration of the measurementbeing controlled based on the counted number of clock cycles.

According to an embodiment, the duration of the measurement is equal toa duration of a first number of successive clock cycles, the firstnumber being lower than the counted number of clock cycles.

According to an embodiment, the first signal is generated based on anoutput signal of at least one pixel of a plurality of pixels of theambient light sensor.

According to an embodiment, the threshold intensity value is determinedsuch that the second signal is in the first state during each secondphase, and in the second state during each first phase.

One embodiment provides an ambient light sensor configured to performthe described method.

One embodiment provides an electronic device comprising: a screen; ascreen driver configured to control the screen to alternate betweenfirst phases in which light is emitted by the screen and second phasesin which no light is emitted by the screen; and the ambient light sensordefined above disposed such that a part of the light emitted by thescreen is received by the ambient light sensor.

According to an embodiment, the ambient light sensor is disposed at afirst side of the screen opposite a second side of the screen from whichthe light is emitted, and wherein, preferably, a transmittance of theambient light through the screen is in a range from 0.5% to 5% when nolight is emitted by the screen.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and advantages, as well as others, will bedescribed in detail in the following description of specific embodimentsgiven by way of illustration and not limitation with reference to theaccompanying drawings, in which:

FIG. 1 illustrates, in front and cross-section views, an embodiment ofan electronic device;

FIG. 2 illustrates, in front and cross-section views, a furtherembodiment of an electronic device;

FIG. 3 schematically illustrates, in the form of blocks, a light sensorof the device of FIG. 1 or 2 ;

FIG. 4 is a timing diagram illustrating a mode of operation of thesensor of FIG. 3 according to an embodiment;

FIG. 5 is a timing diagram illustrating a mode of operation of thesensor of FIG. 3 according to a further embodiment;

FIG. 6 is a timing diagram illustrating a mode of operation of thesensor of FIG. 3 according to yet a further embodiment;

FIG. 7 schematically illustrates, in the form of blocks, a circuitaccording to an example embodiment; and

FIG. 8 is a timing diagram illustrating a mode of operation of thecircuit of FIG. 7 .

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Like features have been designated by like references in the variousfigures. In particular, the structural and/or functional features thatare common among the various embodiments may have the same referencesand may dispose identical structural, dimensional and materialproperties.

For the sake of clarity, only the operations and elements that areuseful for an understanding of the embodiments described herein havebeen illustrated and described in detail.

Unless indicated otherwise, when reference is made to two elementsconnected together, this signifies a direct connection without anyintermediate elements other than conductors, and when reference is madeto two elements coupled together, this signifies that these two elementscan be connected or they can be coupled via one or more other elements.

In the following disclosure, unless indicated otherwise, when referenceis made to absolute positional qualifiers, such as the terms “front”,“back”, “top”, “bottom”, “left”, “right”, etc., or to relativepositional qualifiers, such as the terms “above”, “below”, “higher”,“lower”, etc., or to qualifiers of orientation, such as “horizontal”,“vertical”, etc., reference is made to the orientation shown in thefigures.

Unless specified otherwise, the expressions “around”, “approximately”,“substantially” and “in the order of” signify within 10%, and preferablywithin 5%.

In the following disclosure, electronic systems are considered in whichthe screen operates by alternating phases in which the screen emitslight and phases in which the screen is turned off, i.e. the screenemits no light. In such systems, the average light power emitted by thescreen and perceived by a user is adapted by modifying the duty cycleand/or the frequency of the screen activation, for example by adjustingthe duration of the phases of light emission and/or the duration of thephases in which no light is emitted. With adequate switching frequenciesbetween the phases in which the screen emits light and the phases inwhich the screen is turned off, the user of the screen does not perceivethe transitions between these phases, due to the persistence of visionof the human eye. For example, the switching frequency is at least 25Hz.

For instance, the screen is controlled by a binary control signal, afirst state of which causes a phase of light emission by the screen, anda second state of which causes a phase in which no light is emitted bythe screen. This control signal generally undergoes pulse-widthmodulation (PWM) or pulse-frequency modulation (PFM). The type ofscreen, for example LCD (Liquid Crystal Display) or OLED (Organic LightEmitting Diode), to which such control modes apply and the manner ofimplementation of these control modes are not described in detail. Thedescribed embodiments are compatible with these known control modes andthe known screens to which these control modes apply.

FIG. 1 illustrates two views A and B of an embodiment of an electronicdevice 2000, in this example a mobile telephone 2000, the view A being afront view of the telephone 2000 and the view B being a cross-sectionview along the plane BB indicated in view A.

The device 2000 comprises an electronic system or circuit 1000. Theelectronic circuit 1000 comprises a screen 100 configured to displayimages and/or information destined for a user. The display screen, orpanel, 100 comprises a matrix of light emitting pixels (notillustrated).

The system 1000 further comprises various electronic circuits includingan ambient light sensor 104. In the example shown in FIG. 1 , in theview B, two further electronic circuits, namely a processing unit 106and a driver or control circuit 108 of the screen 100, are illustrated.

The various electronic circuits of the system 1000 are, for example,mounted on a printed circuit board (PCB) 110, preferably a flexibleprinted circuit board, in order to be electrically coupled with oneanother via the board 110. Although a single board 110 is illustrated inthe view B shown in FIG. 1 , the system 1000 can comprise a plurality ofboards 110, possibly electrically coupled with one another via ribboncables.

For instance, the display screen 100 can be of the OLED type (OrganicLight Emitting Diode). The screen 100 is thus, for example, controlledby a binary control signal, for example generated by the driver 108.This control signal is, for example, provided selectively to each diodeof the screen, so as to cause alternate phases in which at least certaindiodes of the screen 100 emit light and phases in which no diode of thescreen 100 emits light. The selection of the diodes of the screen 100receiving or not receiving the control signal is, for example,implemented by the driver 108. In certain cases, the driver 108 canfurther adapt, for each diode, the voltage level of the binary signalcorresponding to a phase of light emission so as to adapt the lightpower emitted by the diode. Each pixel of the screen can consist of oneor more diodes, possibly covered by an RGB (Red, Green, Blue) colorfilter.

For instance, the display screen 100 can also be of the LCD type (LiquidCrystal Display). The screen 100 thus comprises, for example, a matrixof pixels each comprising polarizing liquid crystal filters, and anilluminating plate or panel disposed under the matrix of pixels. Theplate is, for example, controlled by a binary control signal, forexample generated by the driver 108, so that the plate operates byalternating phases of light emission and phases in which the plate doesnot emit any light. In certain cases, the driver 108 can further adaptthe voltage level of the binary signal corresponding to a phase of lightemission so as to adapt the light power emitted by the plate. Thepolarizing filters of each pixel are controlled, for example by thedriver 108 of the screen 100, to allow or prevent the light emitted bythe plate to pass through the polarizing filters, towards a user. Eachpixel of the screen can be covered by one or more RGB color filters.

In the illustrated example, the system 1000 further comprises, above thedisplay screen 100, a touch screen or touch plate 112. The touch screen112 entirely covers the display screen 100, the screens 100 and 112having substantially the same surface areas, preferably the same surfaceareas.

Typically, the device 2000 comprises a protective glass pane 114covering the screen 100, and, more specifically in this example, theassembly comprising the two screens 100 and 112. The glass pane 114entirely covers the screen 100, the surface area of the glass pane 114being substantially equal to that of the screen 100, preferably equal tothat of the screen 100.

The device 2000 comprises a housing, or shell, 116, in which the system1000 is disposed, i.e. in which the electronic circuits 104, 106 and 108and the one or more boards 110 are disposed. The assembly of the screen100, the optional touch screen 112 and the glass pane 114 closes thehousing 116 on the side of a face of the system, the upper face in theview B of FIG. 1 , and the face that is visible in the view A of FIG. 1.

In this embodiment, the telephone 2000 is called “borderless”, i.e. thescreen 100, and more specifically the assembly of the screen 100, theoptional touch screen 112 and the glass pane 114, occupies substantiallythe entire face, preferably the entire face, of the device intended tobe viewed by the user of the system, i.e. the upper face of the device2000 in the view B of FIG. 1 . The ambient light sensor 104 is thusdisposed under the screen 100, i.e. on the side of the screen 100opposite the face of the screen 100 from which light is emitted by thescreen. The display screen 100, the optional touch screen 112 and theglass pane 114 are thus at least partially transparent to the ambientlight, the ambient light for example corresponding here to the visiblelight and possibly to infra-red and/or ultra-violet light. Thus, ambientlight can pass through the assembly of the glass pane 114, the optionaltouch screen 112 and the display screen 100, and reach the sensor 104.

FIG. 2 illustrates two views A and B of a further embodiment of anelectronic device 3000, in this example a mobile telephone 3000, theview A being a front view of the telephone and the view B being across-section view along the plane BB indicated in view A.

The device 3000 of FIG. 2 differs from the device 2000 of FIG. 1 in thatthe display screen 100 and the optional touch screen 112 are interruptedabove the sensor 104 in order to allow the ambient light to reach thesensor 104. More specifically, a window, or a notch, 118 is providedthrough the screen 100 and the optional touch screen 112, above thesensor 104. The glass pane 114 covers the window 118 so as to protectthe electronic circuits disposed in the housing 116, and in particularthe sensor 104.

For example, in the devices 2000 and 3000, during a phase in which thescreen is turned off, and, thus, no light is emitted by the screen, thetransmittance of the light through the assembly of the glass pane 114,the optional touch screen 114 and the display screen 112 is in the rangefrom 0.5% to 5%.

It should be noted that the devices 2000 and 3000 are illustrated in avery schematic fashion, and that not all details of these devices havebeen illustrated. The embodiments that will be described in thefollowing are not limited to the example devices shown in FIGS. 1 and 2, but apply to all electronic devices comprising an electronic system1000, for example tablet computers, connected watches, computer screens,mobile telephones, multimedia apparatus equipped with, for example, aflexible or foldable screen, etc. More specifically, the describedembodiments apply to electronic systems 1000 comprising a display screen100 and an ambient light sensor 104 disposed under the screen 100 asillustrated in FIG. 1 , or under a window, or opening, 118 of the screen100 as illustrated in FIG. 2 , in which the screen 100 operates byalternating phases of light emission and phases in which no light isemitted.

FIG. 3 schematically illustrates, in the form of blocks, the lightsensor 104 of the device of FIG. 1 or 2 .

The ambient light sensor 104 for example comprises a light sensitivearea 1041 comprising at least one pixel (not shown) receiving light.Preferably, the area 1041 comprises more than one pixel. Each pixel ofthe area 1041 provides an output signal 1045.

For instance, the output signal 1045 of a pixel is an analog signal, thevalue of which being for example representative of a value of aphotocurrent generated by a photodiode of the pixel. Although this isnot shown, the output signal of the pixel is for example provided by atransimpedance amplifier; a source follower transistor having its gatecoupled to a buffering capacitor in which a voltage representative ofthe charges photogenerated in the photodiode are stored; or a chargeintegrator converting the photogenerated charges in the photodiode intoa rising voltage. The output signal could also be provided by a readoutcircuit of the pixel as for example described in the application U.S.Pat. No. 9,927,291, the content of which being hereby incorporated byreference in its entirety.

The output signal 1045 of a pixel could also be a binary signal having apulse each time a photon received by a single-photon avalanche diode(SPAD) of the pixel triggers an avalanche phenomenon, or a voltagehaving a value representative of the number of pulses generated for agiven time period.

The light sensor 104 comprises a read circuit 1043. The read circuit1043 is configured to receive the output signals 1045 of the pixels ofthe area 1041. In this embodiment, the read circuit 1043 is furtherconfigured to provide, or generate, an output signal OUT of the sensor104, based on the signals 1045. The signal OUT, which is for example adigital signal comprising a plurality of bits, is representative of theamount of light received by the area 1041 during a given measurementphase, and thus of the amount of light received by the sensor 104 duringthis given measurement phase.

For instance, all the pixels of the sensor 104 are configured to receivelight in a same and single wavelength range, and the signal OUT isrepresentative of the amount of light received during a measurementphase in this single wavelength range.

The pixels of the sensor 104 could also be separated in a plurality ofsets of pixels such that, in each set of pixels, the pixels of the setare configured to receive light only in a given corresponding range ofwavelengths, different from the ranges of wavelength of the other setsof pixels, for example by associating each pixel of each set of pixelswith corresponding filters adapted to the range of wavelengths of thisset of pixels. The signal OUT can thus be representative of the amountof light received during a measurement phase, in each of the pluralityof wavelength ranges corresponding to the plurality of sets of pixels.With such an signal OUT, the system 1000 (FIG. 1 or 2 ) can beconfigured to determine the type of ambient light, for example if thelight is natural, from a filament light bulb, from a fluorescent lightbulb, if the light is a cold or warm light, etc., based on the spectralrepartition of the light between each of the plurality of wavelengthranges. Furthermore, in the case where the screen 100 is a colour screenof the OLED type, the processing unit 106 and/or the driver 108 (FIG. 1or 2 ) can thus be configured to control the screen 100 such that, foreach wavelength range that the screen 100 can emit, the screen 100receives an indication of the average target power that the screen 100needs to emit for this wavelength range. Indeed, in the case of an OLEDcolour screen, the circuit 108 is generally configured to control eachpixel of the screen individually. As a result, the system 1000 can thusadapt the type of light emitted by its screen 100 to the type of ambientlight which surrounds the system 1000.

The read circuit 1043 is further configured to provide, or generate, asignal L_int based on the signals 1045 coming from at least certainpixels of the area 1041, the signal L_int being preferably an analogsignal. The signal L_int is representative of the intensity of lightreceived by the sensor 104, and, more precisely, by the area 1041 of thesensor 104, during the sensor 104 operation. In other words, the valueof the signal L_int changes when the intensity of the light received bythe sensor 104 changes. For example, the value of the signal L_int isupdated such that a time period between each two successive values ofthe signal L_int is at least 10 times, preferably 100 times, shorterthan the minimal time duration of the phases in which no light isemitted by the screen 100.

For instance, in the case where each signal 1045 is a photocurrentgenerated by one of the photodiodes of the pixels of the area 1041, thevalue of signal L_int can correspond to a mean value of the values ofthe signals 1045.

For instance, in the case where each signal 1045 presents a pulse foreach avalanche phenomenon occurring in a SPAD of the pixel correspondingto the signal 1045, the value of the signal L_int can be representativeof the number of pulses received by the read circuit 1043 for a giventime duration among successive periodic time durations. In such a case,the value of the signals 1045, and thus the value of the signal L_int,is updated at the end of each of these successive periodic timedurations.

The read circuit 1043 is under the control of a binary control signalMES, such that, for each ambient light measurement phase, a start and anend of the measurement phase, and thus its duration, are determinedbased on the signal MES. The circuit 1043 comprises an input configuredto receive the signal MES. Each time the signal MES is in a first state,for example a high state, an ambient light measurement phase isperformed by the sensor 104. The start of each ambient light measurementis triggered by a transition of the signal MES to its first state, andthe end of the ambient light measurement is triggered by a nexttransition of the signal MES to its second state, for example a lowstate.

The sensor 104 further comprises a control circuit 1047. The controlcircuit 1047 comprises an input configured to receive the signal L_int,and an output configured to provide the signal MES. The control circuit1047 is configured to generate, or provide, the signal MES based on thesignal L_int. Thus, the circuit 1047 controls, by mean of signal MES,timings of each ambient light measurement phase, that is to say thestart and the end of the ambient light measurement phase. The circuit1047 is configured such that the ambient light measurement occurs duringa phase in which no light is emitted by the screen 100.

For example, the circuit 1047 is configured to compare the value of thesignal L_int with a threshold intensity value th. The circuit 1047 is,for example, further configured to generate a binary signal COMPrepresentative of the result of the comparison of the signal L_int withthe threshold intensity value th. The threshold intensity value th ischosen such that the state of the signal COMP indicates whether thescreen 100 is emitting light or not.

A first state, for example a high state, of the signal COMP indicatesthat the value of intensity of the light received by the sensor 104 isbelow a threshold determined based on the threshold intensity value th,the signal COMP being in its first state when, for example, the signalL_int is below the threshold th. A second state, for example a lowstate, of the signal COMP indicates that the value of intensity of thelight received by the sensor 104 is above the threshold determined basedon the threshold intensity value th, the signal COMP being in its secondstate when, for example, the signal L_int is above the threshold th. Thethreshold intensity value th is chosen such that the first state of thesignal COMP indicates that no light is emitted by screen 100, and thesecond state of the signal COMP indicates that screen 100 is emittinglight and that a part of this emitted light is received by the sensor104.

For example, the circuit 1047 comprises a circuit, or comparator, 1049configured to compare the signal L_int with the threshold intensityvalue th, and generate the COMP signal accordingly. For example, thecomparator 1049 comprises a first input, for example a negative input(−), configured to receive the signal L_int, a second input, for examplea positive input (+), configured to receive the threshold intensityvalue th, and an output configured to generate the COMP signal.

The circuit 1047 is then configured to generate the signal MES based onthe signal COMP, such that an ambient light measurement is performedduring a corresponding phase in which no light is emitted by the screen100. By doing this, value of the measured ambient light is not distortedby the light emitted by the screen 100 to the sensor 104 during thephases in which light is emitted by the screen 100.

FIG. 4 is a timing diagram illustrating a mode of operation of thesensor 104 of FIG. 3 according to an embodiment. In the embodiment ofFIG. 4 , the signal MES is identical to the signal COMP.

At an instant to, the screen 100 is controlled to be in a phase ON inwhich light is emitted by screen 100. As a part of the emitted light isreceived by the sensor 104, the signal L_int has a value superior to thethreshold intensity value th. Thus, the signal COMP and the signal MES,which are here identical to each other, are in their second states, inthis example low states.

The screen 100 is in the ON-phase until an instant t1 posterior to theinstant to. At the instant t1, the screen 100 is switched to a phase OFFin which no light is emitted by the screen 100. As no light emitted bythe screen can be received by the sensor 104, the value of the signalL_int decreases and becomes inferior to the threshold intensity valueth. Thus, the signals COMP and MES transition to their first states, inthis example high states.

The screen 100 is in the OFF-phase until an instant t2 posterior to theinstant t1. At the instant t2, the screen is switched to an ON-phase.The value of the signal L_int increases and becomes superior to thethreshold intensity value th. Thus, the signals COMP and MES transitionto their second states.

The screen 100 is in the ON-phase until an instant t3 posterior to theinstant t2. At the instant t3, the screen is switched to an OFF-phase,the signal L_int drops below the threshold intensity value th, and thesignals COMP and MES transition to their first states.

The screen 100 is in the OFF-phase until an instant t4 posterior to theinstant t3. At the instant t4, the screen is switched to an ON-phase,the signal L_int goes above the threshold intensity value th, and thesignals COMP and MES transition to their second states.

The screen 100 is in the ON-phase until an instant t5 posterior to theinstant t4. At the instant t5, the screen is switched to an OFF-phase,the signal L_int drops below the threshold intensity value th, and thesignals COMP and MES transition to their first states.

In the embodiment of FIG. 4 , each transition of the signal MES to itsfirst state triggers a start of a corresponding ambient lightmeasurement phase (instants t1, t3 and t5), and each transition of thesignal MES to its second state ends a corresponding ambient lightmeasurement phase (instants t2 and t4). In other words, each ambientlight measurement is performed from a transition of the signal COMP toits first state, until the immediately following or next transition ofthe signal COMP to its second state.

Thus, in the embodiment of FIG. 4 , the duration of an ambient lightmeasurement phase triggered by a given transition of the signal MES toits first state is determined by a corresponding duration of the firststate of the signal MES, that is to say the duration of the first stateof the signal MES following this given transition. In this way, eachambient light measurement phase is performed during a correspondingOFF-phase.

Although three successive ON-phases, which alternate with threesuccessive OFF-phases, have been represented in FIG. 4 , the sensor 104is configured to operate in the manner described above whatever thenumber of alternating ON-phases and OFF-phases.

FIG. 5 is a timing diagram illustrating a mode of operation of thesensor 104 of FIG. 3 according to a further embodiment.

More particularly, in this embodiment, it is considered that, when thescreen 100 switches from an ON-phase to an OFF-phase, due to theafterglow of the screen 100, the intensity of the light emitted by thescreen decreases progressively between a high value at the end of theON-phase and a null value, which is reached during the OFF-phase. Thus,the transition of the signal COMP may occur even when the intensity ofthe light emitted by the screen is not yet null.

In this embodiment, an ambient light measurement phase starts a giventime delay Ts after a transition of the signal COMP to its first state,the time delay Ts being chosen such that, at the start of the ambientlight measurement, the intensity of light emitted by the screen 100 isnull. Thus, the time delay is determined based on the duration of theafterglow of the screen. For example, the time delay Ts is in the range0.1 μs to 100 μs.

In FIG. 5 , as in FIG. 4 , at the instant to, the screen 100 is in anON-phase, the signal L_int being thus above the threshold intensityvalue th, and the signals COMP and MES being in their second states.

In FIG. 5 , as in FIG. 4 , at the instant t1, the screen 100 is switchedto an OFF-phase. The intensity of the light emitted by the screen 100then decreases progressively to reach a null value at an instant t1_2posterior to the instant t1. As a consequence, the signal L_intdecreases from the instant t1 to the instant t1_2, and drops below thethreshold intensity value th at an instant t1_1 between the instants t1and t1_2. It results that signal COMP transitions to its first state atthe instant t1_1, whereas the intensity of the light emitted by thescreen 100 is not yet null. However, the signal MES is transitioned toits first state, by circuit 1047, the time delay Ts after the instantt1_1, at an instant t1_3 posterior to the instant t1_2 and equal tot1_1+Ts. Thus, when an ambient light measurement phase starts at theinstant t1_3 because of the transition of the signal MES to its firststate, the intensity of the light emitted by the screen is null.

At the instant t2 posterior to the instant t1_3, the screen 100 isswitched to an ON-phase, the signal L_int goes above the thresholdintensity value th, and the signals COMP and MES transition to theirsecond states.

At the instant t3 posterior to the instant t2, the screen 100 isswitched to an OFF-phase. The sensor 104 then operates at successiveinstants t3_1, t3_2 and t3_3 in a manner identical to that described inrelation with the respective instants t1_1, t1_2 and t1_3, the instantt3_3 being thus equal to t3_1+Ts.

At the instant t4 posterior to the instant t3_3, the screen 100 isswitched to an ON-phase, the signal L_int goes above the thresholdintensity value th, and the signals COMP and MES transition to theirsecond states.

At the instant t5 posterior to the instant t4, the screen 100 isswitched to an OFF-phase. The sensor 104 then operates at successiveinstants t5_1, t5_2 and t5_3 in a manner identical to that described inrelation with the respective instants t1_1, t1_2 and t1_3, the instantt5_3 being thus equal to t5_1+Ts.

Although three successive ON-phases, which alternate with threesuccessive OFF-phases, have been represented in FIG. 5 , the sensor 104is configured to operate in the manner described above whatever thenumber of alternating ON-phases and OFF-phases.

In the embodiments of FIGS. 4 and 5 , each transition of the signal COMPto its first state triggers, possibly after the time delay Ts, acorresponding ambient light measurement, and the next transition of thesignal COMP to its second state ends the measurement phase. Thus, theduration of the measurement phase is determined by the duration of thefirst state of the signal COMP following the transition of the signalCOMP which triggers the measurement phase.

In alternative embodiments, each transition of the signal COMP to itsfirst state triggers the start of a corresponding ambient lightmeasurement phase, but the duration of this measurement phase iscontrolled based on at least one duration of the first state of thesignal COMP occurred before this measurement phase. In other words, theduration of this measurement phase is controlled based on at least oneduration of the first state of the signal COMP that occurred before thetransition of the signal COMP that triggered this measurement phase.

FIG. 6 is a timing diagram illustrating a mode of operation of thesensor 104 of FIG. 3 according to a further embodiment, in which theduration of an ambient light measurement phase is determined based on atleast one previous duration of the first state of signal COMP, in thisexample by the immediately preceding duration of the first state ofsignal COMP.

In FIG. 6 , as in FIGS. 4 and 5 , at the instant to, the screen 100 isin an ON-phase, the signal L_int being thus above the thresholdintensity value th, and the signals COMP and MES being in their secondstates. Furthermore, an information representative of the duration T0(not shown on FIG. 6 ) of the last first state of the signal COMP beforethe instant to has been stored by the circuit 1047, for example in amemory of the circuit 1047.

At the instant t1, the screen 100 is switched to an OFF-phase, and thesignal L_int decreases progressively to reach a null value at theinstant t1_2, the signal L_int dropping below the threshold intensityvalue th at the instant t1_1 between the instants t1 and t1_2.

Because the signal L_int drops below the threshold th at the instantt1_1, the signal COMP transitions to its first state at the instantt1_1. The transition of the signal COMP to its first state causes thesignal MES to transition to its first state, in this example at theinstant t1_3 separated from the instant t1_1 by the time delay Ts.Furthermore, the signal MES is maintained at its first state for aduration T0′ inferior to the duration T0, such that the signal MEStransitions to its second state, while the screen 100 is still in theOFF-phase. Thus, during the OFF-phase between the instants t1 and t2,the duration of the measurement phase performed is thus T0′.

In this example, the duration T0′ is equal to T0−Ts−Te, Te being a giventime duration. The time duration Te is chosen such that, at the end ofthe time duration T0′, when the signal MES is transitioned to its secondstate at an instant t1_4 equal to t1_3+T0′, the screen 100 is still inthe OFF-phase.

The subtraction of the duration Te from the previous duration T0 allowsto ensure that the ambient light measurement phase ends before the nextON-phase in which light is emitted by screen, even in case where thesignal COMP transition to its second state with a delay compared to theinstant at which the signal L_int goes above the threshold intensityvalue th.

Moreover, between the instants t1 and t2, the circuit 1047 obtains orgenerates an information representative of a duration T1 of the firststate of the signal COMP between these instants, and stores thisinformation.

At the instant t3, the screen 100 is switched to an OFF-phase, and thesignal L_int decreases progressively to reach a null value at theinstant t3_2, the signal L_int dropping below the threshold intensityvalue th at the instant t3_1 between the instants t3 and t3_2. Thus, thesignal COMP transitions to its first state at the instant t3_1, and thesignal MES is transitioned to its first state, in this example at theinstant t3_3 equal to t3_1+Ts. The signal MES is then maintained at itsfirst state for a duration T1′ inferior to the duration T1, at least bythe time duration Te, and, more precisely in this example, for a timeduration T1′ equal to T1−Ts−Te. The signal MES is transitioned to itssecond state at an instant t3_4 equal to t3_3+T1′, while the screen 100is still in the OFF-phase, which ends at the instant t4 posterior to theinstant t3_4. Thus, during the OFF-phase between the instants t3 and t4,the duration of the measurement phase performed is T1′.

Moreover, information representative of a duration T2 of the first stateof signal COMP between the instants t3_1 and t4 is stored by the circuit1047.

At the instant t5, the screen 100 is switched to an OFF-phase, and thesignal L_int decreases progressively to reach a null value at theinstant t5_2, the signal L_int dropping below the threshold intensityvalue th at the instant t5_1 between the instants t5 and t5_2. Thus, thesignal COMP transitions to its first state at the instant t5_1, and thesignal MES is transitioned to its first state, in this example at theinstant t5_3 equal to t5_1+Ts. The signal MES is then maintained at itsfirst state for a duration T2′ inferior to the duration T2, at least bythe time duration Te, and, more precisely in this example, for a timeduration T2′ equal to T2−Ts−Te. The signal MES is transitioned to itssecond state at an instant (not shown) equal to t5_3+T2′, while thescreen 100 is still in the OFF-phase. Thus, the duration of themeasurement phase performed during the OFF-phase starting at the instantt5 is T2′.

Moreover, information representative of a duration T3 of the first stateof signal COMP starting at the instant t5_1 is stored by the circuit1047.

Although three successive ON-phases, which alternate with threesuccessive OFF-phases, have been represented in FIG. 6 , the sensor 104is configured to operate in the manner described above whatever thenumber of alternating ON-phases and OFF-phases.

In an alternative embodiment, the duration of an ambient lightmeasurement phase following a given transition of the signal COMP to itsfirst state is determined based on several previous durations of thefirst state of the signal COMP. For example, the duration T2′ of theambient light measurement phase following the transition of signal COMPto its first state at the instant t5 is determined based on thedurations T0, T1 and T2, the duration T2′ being for example equal toTmean−Te−Ts, with Tmean the mean value of durations T0, T1 and T2.

In the embodiment of FIG. 6 , the information representative of aduration of the first state of the signal COMP is for example output bya counter, which is configured to count a number of cycles of a periodicsignal, for example, a clock signal, while the signal COMP is in itsfirst state. The period of the periodic signal is, for example, at least10 times shorter than the minimal duration of an OFF-phase, preferablyat least 100 times shorter than the minimal duration of an OFF-phase.The duration of a measurement phase could then correspond to theduration of a given number of successive cycles of the periodic signal,for example a number of successive cycles equal to the number of cyclescounted during the previous first state of signal COMP, from which anumber of cycles corresponding to the time duration Te, and, possibly, anumber of cycles corresponding to the time duration Ts have beensubtracted. Thus, when signal COMP switches to its first state, thesignal MES is switched to its first state after a number of cyclescorresponding to the time duration Te, the signal MES being thenswitched to its second state at the end of the duration of themeasurement phase, which was determined as described above.

In the embodiment of FIG. 6 , the signal MES is for example generated byusing at last one phase-locked loop of the circuit 1047.

For example, one phase-locked loop is locked on the transitions of thesignal COMP to its first state, and one phase-locked loop is locked onthe transitions of the COMP signal to its second state, the outputsignal of the two phase-locked loops being used to generate acorresponding signal MES. In this case, the information representativeof the duration of the first state of the signal COMP corresponds to aphase difference between the two outputs of the phase-locked loops, andis stored inside the circuit 1047 by the phase-locked loops themselves.

As another example, a single phase-locked loop is locked on transitionsof the signal COMP to its second state. For example, the phase lockedloop comprises a voltage-controlled oscillator, the output signal ofwhich being delayed by the time duration Te, and a phase detectoroutputting a signal representative of the phase difference betweentransitions of the delayed signal to its second state and transitions ofthe COMP signal to its second state. The output signal of the phasedetector is used, possibly after being filtered by a low-pass filter, tocontrol the voltage-controlled oscillator, thus to control the phaseand/or the frequency of the output signal of the voltage-controlledoscillator. The transitions of the signal COMP to its first state areused, preferably after being delayed by the time duration Ts, to switchthe signal MES to its first state, and the transitions of the outputsignal of the voltage-controlled oscillator to its second state are usedto switch the signal MES to its second state. For example, the delayedsignal COMP is provided to a set input of a RS type latch, the outputsignal of the voltage-controlled oscillator is provided to a reset inputof the latch, and the signal MES is provided by an output of the latch.

FIG. 7 illustrates, in the form of blocks, a circuit configured togenerate the signal MES based on the COMP signal using a singlephase-locked loop, according to an example embodiment of the typedescribed above.

The phase-locked loop PLL comprises a voltage-controlled oscillator VCO,a delay circuit eD, a phase shift detector circuit PSD, and, in thisexample, a low-pass filter LPF.

The oscillator VCO provides a MESe signal to the delay circuit eD.

The delay circuit eD applies a delay equal to the time duration Te tothe signal MESe, the resulting delayed signal MESed being provided bythe circuit eD.

The circuit PSD receives the MESed signal and the COMP signal andprovides a signal PSDo representative of the phase shift between signalsMESed and COMP.

The circuit VCO is controlled based on the signal PSDo such that, instationary operation, the switching of the signal MESed to its secondstate are synchronized with the switching of the COMP signal to itssecond state. More particularly, in this example, the signal PSDo isprovided to the filter LPF, and the resulting filtered signal fPSDo isthe control signal of the circuit VCO.

Moreover, the COMP signal is delayed by the time duration Ts by a delaycircuit sD which provides a delayed signal MESs.

The signal MES is then generated based on the MESe and MESs signals. Inthis example, this is done using a RS type latch RS, an output of whichproviding the MESe signal. A reset input of the latch RS receives thesignal MESe inverted by an inverter circuit INV, a set input of thelatch RS receiving the signal MESs.

FIG. 8 is a timing diagram illustrating a mode of operation of thecircuit of FIG. 7 . FIG. 8 shows the evolution of signals COMP, MESs,MESe, MESed and MES.

As it can be seen, the signal MES switches to its first state when thesignal MESs, thus the signal COMP delayed by the time duration Ts,switches to its first state, and switches to its second state when thesignal MESe switches to its second state. Thus, the signal MES switchesto its first state with a delay equal to the duration Te after acorresponding switching of the COMP signal to its first state, and toits second state with a time advance equal to the time duration Tebefore a corresponding switching of the signal COMP to its second state.

In another alternative embodiment, a filtering function is applied tothe value of each duration of the first state of the signal COMP toremove values that are relatively far from the previous values, forexample to remove a value that differs from at least one previous valueby more than 10%. This alternative embodiment is compatible with all theembodiments described in relation with FIG. 6 .

In another alternative embodiment, each transition of the signal COMP toits first state is not delayed by the time duration Ts after acorresponding transition of the signal MES to its first state. Thus, inthis case, the time duration Ts is not subtracted from the previousduration of the first state of signal COMP. This alternative embodimentis compatible with all the embodiments described in relation with FIG. 6.

In another alternative embodiment, a value representative of the amountof light received by the sensor 104 during a given ambient lightmeasurement phase is weighted by the duration of this measurement phase,for example, by dividing this value by the duration of the measurementphase or by a number of cycles of a periodic signal representative ofthis duration. The signal OUT provided at the end of the measurement isthen representative of the weighted value. This allows the ambient lightto be measured without being sensitive to the fact that differentambient light measurement phases could have different durations. Thisalternative embodiment is compatible with all the embodiments describedin relation with FIGS. 4,5 and 6 .

In another alternative embodiment, the circuit 1047 is furtherconfigured to detect when a current ambient light measurement phase endsduring on ON-phase of the screen 100, for example by comparing thestates of signals COMP and MES and detecting when the signal MES is atits first state while the signal COMP is at its second state. The resultof this detection can be output by the sensor 104, for example in orderto validate or invalidate an output value of the signal OUT. Thegeneration of an output value of the signal OUT could also beconditioned by the results of this detection, in order to ensure thatonly values of the signal OUT corresponding to ambient lightmeasurements integrally performed during corresponding OFF-phases areoutput. This alternative embodiment is compatible with all theembodiments described in relation with FIG. 6 .

As an example, in the embodiments described above, each ON-phase has aduration comprised between 1 ms and 10 ms, and each OFF-phase has aduration comprised between 50 μs and 500 μs.

The embodiments described above in relation with FIGS. 3, 4, 5, 6, 7 and8 allow to synchronize an ambient light measurement phase with acorresponding OFF-phase of a screen alternating between OFF-phases andON-phases. This synchronization is done without the use of a signalcontrolling the switching of the screen between the OFF-phases and theON-phases, or a signal derived from this control signal. Compared to theembodiments described above, the use of such a signal would require anadditional input pin for the sensor 104, and would lead to issuesbecause of the unknown and uncontrolled time delay occurring in thesesignals.

It is particularly interesting to implement the above-describedembodiments in electronic devices comprising an ambient light sensor 104disposed below a screen 100 and without a notch or window above thelight sensor 104. However, the embodiments described here could also beimplemented in the case where the screen 100 comprises such a notch orwindow above the sensor 104. Indeed, even with such a notch or window,an ambient light measurement at least partly performed during anON-phase of the screen will be distorted by the light emitted by thescreen 100.

Various embodiments and variants have been described. Those skilled inthe art will understand that certain features of these embodiments canbe combined and other variants will readily occur to those skilled inthe art. In particular, when it has been indicated that the first state,respectively the second state, of a binary signal corresponds to a highstate, respectively to a low state, of this signal, those skilled in theart are capable of adapting the described embodiments to the case wherethe first state, respectively the second state, of this signalcorresponds to the low state, respectively to the high state, of thissignal. Furthermore, although the value of the signal L_int describedabove increases, respectively decreases, when the intensity of the lightreceived by the sensor 104 increases, respectively decreases, thoseskilled in the art are capable of adapting the described embodiments tothe case where the value of the signal L_int described above increases,respectively decreases, when the intensity of the light received by thesensor 104 decreases, respectively increases.

Finally, the practical implementation of the embodiments and variantsdescribed herein is within the capabilities of those skilled in the artbased on the functional description provided hereinabove. In particular,the implementation of the read circuit 1043 and/or the implementationthe control circuit 1047 and/or the choice of the value Ts and/or thechoice of the value Te are within the capabilities of those skilled inthe art based on the functional description provided hereinabove.

What is claimed is:
 1. A method of measuring ambient light comprising: generating, by an ambient light sensor associated with a screen that alternates between first phases in which light is emitted by the screen, a part of which is received by the ambient light sensor, and second phases in which no light is emitted by the screen, a first signal representative of an intensity of light received by the ambient light sensor during the first and second phases; comparing the first signal with a threshold intensity value, the comparing comprising: generating a second signal having a first state in response to the intensity of the light received by the ambient light sensor being below the threshold intensity value, and a second state in response to the intensity of the light received by the ambient light sensor being above the threshold intensity value; and determining the threshold intensity value such that the second signal is in the first state during each second phase, and in the second state during each first phase; and controlling a timing of an ambient light measurement by the ambient light sensor based on the comparison, a start of the measurement being triggered by at least one transition of the second signal to the first state such that the measurement starts when the second signal is in the first state, and a duration of the measurement is controlled based on at least one duration of the first state of the second signal.
 2. The method according to claim 1, wherein the measurement starts following a first transition of the second signal to the first state.
 3. The method according to claim 2, wherein the measurement starts a time delay between 0.1 us and 100 us after the first transition.
 4. The method according to claim 2, wherein the first transition triggers the start of the measurement, and a next transition of the second signal to the second state following the first transition ends the measurement.
 5. The method according to claim 2, wherein the duration of the measurement is controlled based on at least one previous duration of the first state of the second signal.
 6. The method according to claim 5, wherein the duration of the measurement is controlled to be shorter than the at least one previous duration of the first state of the second signal.
 7. The method according to claim 5, wherein the duration of the measurement is controlled based on an average value of at least two successive previous durations of the first state of the second signal.
 8. The method according to claim 5, wherein the at least one previous duration of the first state of the second signal comprises the previous duration of the first state of the second signal immediately preceding the first state of the second signal during which the measurement starts.
 9. The method according to claim 5, further comprising counting a number of clock cycles with a counter during the at least one previous duration of the first state of the second signal, the duration of the measurement being controlled based on the counted number of clock cycles.
 10. The method according to claim 9, wherein the duration of the measurement is equal to a duration of a first number of successive clock cycles, the first number being lower than the counted number of clock cycles.
 11. The method according to claim 1, wherein the first signal is generated based on an output signal of at least one pixel of a plurality of pixels of the ambient light sensor.
 12. An ambient light sensor associated with a screen that alternates between first phases in which light is emitted by the screen, a part of which is received by the ambient light sensor, and second phases in which no light is emitted by the screen, the ambient light sensor comprising: electronic circuitry configured to: generate a first signal representative of an intensity of light received by the ambient light sensor during the first and second phases; compare the first signal with a threshold intensity value, the compare comprising the electronic circuitry configured to: generate a second signal having a first state in response to the intensity of the light received by the ambient light sensor being below the threshold intensity value, and a second state in response to the intensity of the light received by the ambient light sensor being above the threshold intensity value; and determine the threshold intensity value such that the second signal is in the first state during each second phase, and in the second state during each first phase; and control a timing of an ambient light measurement by the ambient light sensor based on the comparison, wherein a start of the measurement is triggered by at least one transition of the second signal to the first state such that the measurement starts when the second signal is in the first state, and a duration of the measurement is controlled based on at least one duration of the first state of the second signal.
 13. The ambient light sensor of claim 12, wherein the electronic circuitry is configured to start the measurement following a first transition of the second signal to the first state.
 14. The ambient light sensor of claim 12, wherein the first signal is generated based on an output signal of at least one pixel of a plurality of pixels of the ambient light sensor.
 15. The ambient light sensor of claim 12, wherein the measurement starts following a first transition of the second signal to the first state.
 16. An electronic device comprising: a screen; a screen driver configured to control the screen to alternate between first phases in which light is emitted by the screen and second phases in which no light is emitted by the screen; and an ambient light sensor disposed such that a part of the light emitted by the screen is received by the ambient light sensor, the ambient light sensor comprising: electronic circuitry configured to: generate a first signal representative of an intensity of light received by the ambient light sensor during the first and second phases; compare the first signal with a threshold intensity value, the compare comprising the electronic circuitry configured to: generate a second signal having a first state in response to the intensity of the light received by the ambient light sensor being below the threshold intensity value, and a second state in response to the intensity of the light received by the ambient light sensor being above the threshold intensity value; and determine the threshold intensity value such that the second signal is in the first state during each second phase, and in the second state during each first phase; and control a timing of an ambient light measurement by the ambient light sensor based on the comparison, wherein a start of the measurement is triggered by at least one transition of the second signal to the first state such that the measurement starts when the second signal is in the first state, and a duration of the measurement is controlled based on at least one duration of the first state of the second signal.
 17. The electronic device of claim 16, wherein the ambient light sensor is disposed at a first side of the screen opposite a second side of the screen from which the light is emitted.
 18. The electronic device of claim 17, wherein a transmittance of ambient light through the screen is in a range from 0.5% to 5% when no light is emitted by the screen.
 19. The electronic device of claim 16, wherein the electronic circuitry is configured to start the measurement following a first transition of the second signal to the first state.
 20. The electronic device of claim 16, wherein the first signal is generated based on an output signal of at least one pixel of a plurality of pixels of the ambient light sensor.
 21. The electronic device of claim 16, wherein the measurement starts following a first transition of the second signal to the first state. 